1. FIELD OF THE INVENTION
This invention relates to silicone rubber compositions, and more particularly it relates to such silicone rubber compositions containing a (O) valent platinum-(III) phosphor complex which acts as a curing catalyst.
This invention also provides a method for curing said silicone rubbers, characterized particularly by the use of steam for effecting such curing.
This invention also provides silicone rubber foam compositions containing, as an inorganic filler, silica obtained from a hydrophobic treatment of a wet process silica.
2. DESCRIPTION OF THE PRIOR ART
There are known two general methods for curing a silicone rubber composition to form an elastomer, namely, a method using an organic peroxide and a method using a platinum compound. The present invention employs the latter method.
Curing of a silicone rubber composition in the presence of a platinum compound can be accomplished by subjecting an organopolysiloxane having olefin-type aliphatic unsaturated bonds and a polysiloxane having Si--H bonds to an addition reaction, according to the following reaction scheme: ##STR1##
However, when chloroplatinic acid or platinum metal is used in such reaction, the reaction progresses even at a relatively low temperature range which is close to room temperature, so that, in most cases, the molecular weight of the silicone rubber composition increases, which causes an undesirable increase of the viscosity. Such increase of molecular weight and viscosity may sometimes change the state of the composition into a useless state before it can actually be put to practical use.
Various methods for controlling the reaction at or around room temperature have been proposed in order to overcome that problem. For instance, Japanese Patent Publication No. 31476/69 proposes the use of an inhibitor comprising an organic compound having acetylene bonds and also having at its .alpha.-position a functional group containing an element such as nitrogen, phosphor and sulfur. Also, in Japanese Patent Publication No. 19193/67, it is attempted to transform platinum into a divalent platinum-(III) phosphor complex and use it as a weak catalyst.
Use of the inhibitor disclosed in the above-mentioned Japanese Patent Publication No. 31476/69 has the theoretical advantage that a desired reaction rate can be selected by adjusting the quantity of the inhibitor added, but in practice, such quantitative adjustment is very difficult, and the addition of too much inhibitor may result in improper or imperfect curing. Further, there are cases in which the inhibitor becomes ineffective due to evaporation thereof, or the inhibitor even acts harmfully to the obtained silicone rubber. On the other hand, the weak catalyst used in said Japanese Pat. Pub. No. 19193/67, even though it makes it possible to provide a certain degree of retarding effect at room temperature, is still unsatisfactory for producing the desired effect. Such catalyst also has the disadvantage that it does not cause the curing to progress even if the composition is heated in a closed system. Another disadvantage of using such a catalyst is that the chlorine contained in the platinum compound causes an undesirable effect on the cured silicone rubber.
Generally, for accomplishing steam vulcanization of silicone rubbers, there are available (a) a method using an organic peroxide catalyst, and (b) a method using a platinum compound for the same purpose. According to the latter method, an organopolysiloxane having olefinic unsaturated bonds and an organopolysiloxane having Si--H bonds are reacted as follows in the presence of a platinum compound: ##STR2## In such reaction, plain platinum metal or chloroplatinic acid is commonly used as the platinum compound employed as catalyst. However, the use of such platinum compound involves a serious problem. That is, if such a compound is used, the reaction can progress easily at a temperature close to room temperature whereby to cause an increase of the molecular weight of the composition, with the result being that the viscosity of the composition rises to such a high level that it can hardly be put to practical use. In addition, such compound may cause tackiness on the surfaces of the silicone rubber compositions after vulcanization.
There are known two general methods for obtaining foamed silicone rubbers, namely, a method in which a blowing agent is added to the silicone rubber composition (see Japanese Patent Publication No. 12675/70), and a method in which a low-boiling material by-produced from a condensation reaction of two polysiloxanes, such as represented by the following formula, is utilized (see Japanese Patent Publication Nos. 9297/58, 8755/69, and 9474/70, and Japanese Patent Application No. 119975/74). EQU SiOH + HSi .sub.catalyst SiOSi + H.sub.2
according to the former method, an azo type blowing agent such as azobisisobutyronitrile, diazoaminobenzene, etc., an N-nitroso type blowing agent such as N,N'-dimethyldinitrosopentamethylenetetramine, or a benzenesulphonylhydrazide type blowing agent is used as an essential ingredient, and in order to obtain a uniform foamed product, it is imperative to properly select the quantity and kind of such a blowing agent used and to uniformly disperse such an agent in the composition. These blowing agents also have a disadvantageous aspect in that they may be decomposed to produce a toxic substance.
According to the latter method, if the number of functional groups in the siloxane that participate in the condensation reaction of the two polysiloxanes are few in the silicone rubber composition, the expansion rate may decrease to such an extent that it is no longer possible to produce the desired silicone rubber foams. This often occurs when using, for example, a polysiloxane having a high degree of polymerization.